During a final process of solidification in continuous casting of steel, the unsolidified part of molten steel (referred to as “unsolidified layer”) is withdrawn in accordance with solidification shrinkage, whereby the unsolidified part of the molten steel flows in the direction of withdrawal of the strand. In the unsolidified layer, solute elements such as carbon (C), phosphorus (P), sulfur (S), and manganese (Mn) are concentrated. When the concentrated molten steel flows in the middle portion of the strand and is solidified in that portion, so-called center segregation occurs. Examples of the causes of the flow of the concentrated molten steel at the end of the solidification include, besides the above-described solidification shrinkage, bulging of the strand between rolls due to molten steel static pressure and misalignment of strand support rolls.
This center segregation impairs the quality of steel products, particularly, thick steel plates. For example, if a line pipe material for oil transportation or natural gas transportation has center segregation, the action of sour gas causes hydrogen induced cracking from the center segregation. Similar problems can occur also in structures including offshore structures, storage tanks, and oil tanks. Steel has therefore been frequently required to be used under severe conditions such as under a low temperature or under a highly corrosive environment, whereby reduction of center segregation in the strand has been becoming increasingly important.
Thus, a large number of countermeasures taken to reduce center segregation in the strand or render center segregation harmless have been developed throughout the procedure from the continuous casting process to the rolling process. One such countermeasure known as being particularly effective in overcoming center segregation is a “solidification-terminal stage soft reduction method”, in which a continuously cast strand containing an unsolidified layer inside is pressed down in a continuous casting machine. The “solidification-terminal stage soft reduction method” is a method in which multiple pressing rolls are arranged at or around the solidification completion position of the strand and a continuously cast strand is gradually pressed down by the pressing rolls at a pressing speed approximately corresponding to the rate of solidification shrinkage to prevent occurrence of voids or flows of concentrated molten steel in the center portion of the strand, whereby the center segregation of the strand is suppressed.
For the solidification-terminal stage soft reduction method to effectively prevent center segregation from occurring, it is important to appropriately determine the start and finish time of a period, within the final solidification period of the strand, during which the strand is being subjected to soft reduction and determine the pressing rate during the soft reduction period. Various types of methods of determining the times and the rate have been developed.
For example, Japanese Unexamined Patent Application Publication No. 8-132203 describes a continuous casting method including subjecting soft reduction to a terminal solidification portion of a continuously cast strand and in which the rate at which the strand is pressed per unit time in a section in which the strand is subjected to soft reduction is determined by the strand surface temperature at the pressing start time and the thickness of the unsolidified layer of the strand at the press position.
Japanese Unexamined Patent Application Publication No. 3-90263 and Japanese Unexamined Patent Application Publication No. 3-90259 each describe a continuous steel casting method while pressing a strand with multiple pairs of rolls within a region from the time point at which a thickness-wise middle portion of the bloom strand has a temperature corresponding to the solid fraction of 0.1 to 0.3 to the time point at which the thickness-wise middle portion has a temperature corresponding to the flow-limit solid fraction. In the method, the speed at which the strand is pressed is further increased toward the downstream side in the casting direction with increasing solid fraction at the strand thickness-wise middle portion.
Japanese Unexamined Patent Application Publication No. 2003-71552 describes a continuous steel casting method with an application of pressing force to the strand being cast. In the method, the pressing conditions are determined or adjusted on the basis of the information of the shape of a cross section of the strand taken perpendicular to the longitudinal direction of the strand and the information of the shape of an unsolidified portion in the cross section.
In the continuous slab-strand casting involving solidification-terminal stage soft reduction method, when the thickness of the cast target strand varies, the time at which the soft reduction is to be started and the time at which the soft reduction is to be finished do not change regardless of the thickness of the strand, whereas the optimum pressing speed in the range in which the pressing force is applied to the strand (referred to as “soft reduction zone”) changes in accordance with the thickness of the strand.
The thickness of the slab strand is determined by the thickness of the rolled steel product and the pressing ratio during rolling required for the specifications of the steel product (strand thickness/steel product thickness). Thus, when new specifications of a steel product are determined, the thickness of the strand is determined in accordance with the specifications. If a strand having the determined thickness has never been cast before with the solidification-terminal stage soft reduction method, there is a need to additionally determine an optimum pressing speed during the soft reduction for the strand thickness. Every time the optimum pressing speed is to be determined, an optimum reduction rate in the soft reduction zone is determined through casting experiments using an actual machine under the settings of various different levels of reduction rate, which requires significant time and cost. Specifically, achievement of a method of simply obtaining an optimum reduction rate in the soft reduction zone in accordance with the thickness of the slab strand has been a challenge.
The “reduction rate” refers to the state of the degree of a roll opening determined such that the distance between opposing rolls (referred to as “the roll gap”) gradually decreases toward the downstream side in the casting direction. The reduction rate is usually expressed by the amount by which the degree of the roll opening decreases per 1 m (mm/m). The value obtained by multiplying the reduction rate (mm/m) by the strand withdrawal speed (m/min) is calculated as the pressing speed (mm/min).
Japanese Unexamined Patent Application Publication No. 8-132203 focuses attention on the unsolidified layer thickness of the strand as an indicator of effectively performing soft reduction. According to Japanese Unexamined Patent Application Publication No. 8-132203, this is based on the finding that the pressing rate determined for the pressing rolls is transmitted in a smaller rate to the interface of solid and liquid phases of the strand (hereinafter the rate is referred to as “pressing efficiency”) on the casting downstream side, that is, with decreasing unsolidified layer thickness of the strand. However, the center segregation becomes apparent in a center region of the strand having an unsolidified layer thickness of approximately 10 mm or smaller. According to the relationship between the unsolidified layer thickness D and the pressing speed required per unit time shown in FIG. 1 of Japanese Unexamined Patent Application Publication No. 8-132203, the difference between a pressing speed required for the unsolidified layer thickness of 10 mm and a pressing speed required for the unsolidified layer thickness of 0 mm is approximately 10% at most. The Example in Japanese Unexamined Patent Application Publication No. 8-132203 describes only the test results for one strand thickness (250 mm). Thus, whether the optimum pressing conditions described in Japanese Unexamined Patent Application Publication No. 8-132203 are also effective for different strand thicknesses remains in question.
In Japanese Unexamined Patent Application Publication No. 3-90263 and Japanese Unexamined Patent Application Publication No. 3-90259, the sizes of the strands used in the test in thickness and width range between three types of 300 mm×500 mm, 162 mm×162 mm, and 380 mm×560 mm. All the strands having the above sizes relate to the soft-reduction casting of a bloom strand. Since a bloom strand has a ratio between the width and the thickness of the cross section taken perpendicular to the withdrawal direction of the strand (width/thickness) smaller than that of a slab strand, pressing efficiency in the soft reduction at the end of the solidification of a bloom strand is smaller than that in a slab strand. Accordingly, the pressing rate increases further toward the end of the solidification. The pressing rate is approximately two to three times as large as that in the slab strand in Japanese Unexamined Patent Application Publication No. 8-132203. Those pressing conditions cannot be directly used in the soft reduction of the slab strand.
In Japanese Unexamined Patent Application Publication No. 8-132203, Japanese Unexamined Patent Application Publication No. 3-90263 and Japanese Unexamined Patent Application Publication No. 3-90259, the reduction rate in the soft reduction zone is varied in the casting withdrawal direction and thus the roll gap of strand support rolls is determined with complexity, entailing complexity of the equipment structure for practice with an actual machine.
Japanese Unexamined Patent Application Publication No. 2003-71552 is directed toward a bloom strand. In Japanese Unexamined Patent Application Publication No. 2003-71552, the soft reduction conditions are determined on the basis of information of the shape of the cross section taken perpendicular to the longitudinal direction of the strand, that is, the width and the thickness of the strand. The soft reduction conditions are determined using the ratio between the width and the thickness of the strand as references and on the basis of the amount of change between the references and the ratio between the width and the thickness of the unsolidified portion of the strand. The soft reduction conditions are not determined by directly using the thickness of the strand. In the bloom strand, an unsolidified layer of the strand can have a flat shape either in the lateral direction or in the vertical direction depending on the cooling ratio between the upper and lower surfaces of the strand inside the continuous casting machine or the cooling ratio between the left and right surfaces of the strand inside the continuous casting machine. Thus, the conditions in Japanese Unexamined Patent Application Publication No. 2003-71552 are determined in the above-described manner for the purposes of enabling an optimum soft reduction in either case.
A slab strand has a far larger long side than a short side and the direction in which the unsolidified layer extends flat does not change. The unsolidified layer is always flat in the lateral direction of the strand. Thus, Japanese Unexamined Patent Application Publication No. 2003-71552 is less useful.
It could therefore be helpful to provide a continuous steel casting method with which soft reduction conditions can be determined in accordance with the thickness of a strand, thereby preventing an occurrence of center segregation in the strand due to an insufficient pressing rate or an occurrence of internal cracks in the strand due to an excessively high pressing rate.